This invention relates to methods for sealing a well using a thermite reaction charge placed or lowered into the well. The invention has remedial and sealing applications for wells used in oil and natural gas production, as well as in other applications including sealing of wells used for underground storage of nuclear waste, sequestration of CO2, and the like.
As used in this document, the term “thermite reaction” is intended to refer to a broad class of chemical reactions which can be defined as an exothermic reaction which involves a metal reacting with a metallic or a non-metallic oxide to form a more stable oxide and the corresponding metal or non-metal of the reactant oxide. This is a form of oxidation-reduction reaction which can be written in a general form as:M+AO→MO+A+ΔH
where M is a metal or an alloy (typically, but not necessarily Aluminum) and A is either a metal or a non-metal, MO and AO are their corresponding oxides, and ΔH is the heat generated by the reaction. Commonly, AO is one of the species of Iron Oxide, such as Fe2O3 or Fe3O4. A typical thermite reaction is of the form 2Al+Fe2O3→2 Fe+Al2O3. The reaction produces a great deal of heat per unit of mass, and can attain a reaction temperature of approximately 3,000° C.
Thermite reactions have many uses, including welding, pyrotechnics, synthesis and processing of materials, and military applications. Background information on thermite reactions is described in the review article of Wang et al., Thermite reactions: their utilization for synthesis and processing of materials, J. Materials Science 28 (1993) pp. 3693-3708; and in Fisher et al., A survey of combustible metals, thermites and intermetallics for pyrotechnic applications, presented at the 32nd AIAA/ASME/SE/ASEE Joint Propulsion Conference, Lake Buena Vista, Fla., Jul. 1-3, 1996. Additional background information is found in Orru et al., Self-propagating thermite reactions: effect of alumina and silica in the starting mixture on the structure of the final products, Metallurgical Science and Technology 15 (1)(1997) pp. 31-38. The entire content of the Wang et al., Fisher et al. and Orru et al. articles is incorporated by reference herein.
Thermite reactions have also been proposed for well sealing application, see published PCT application WO 2013/133583 and US patent application publication 2006/0144591. See also U.S. Pat. No. 6,923,263. Thermites have been applied in the drilling industry for blowout prevention (U.S. Pat. No. 5,159,983), explosive sealing of casing perforations (U.S. Pat. No. 5,613,557), gas generation for downhole tool actuation (U.S. Pat. No. 6,925,937), well perforation and hydrofracturing (US patent application publication 2011/0146519) and downhole bonding of metal members (US patent application publication 2012/0255742). Many other patents exist for welding and demolition with thermite in above ground applications, but these are not considered relevant to borehole seal applications.
The basic concept of emplacing thermite charges into a well to perform sealing and structural roles is depicted in FIGS. 1A-1C. Referring to FIG. 1A, a well 100 (shown in cross-section) defined by the inner walls of a formation 1 has a platform 2 in the form of a backfill or bridge plug inserted into the well. A thermite charge 4, typically a compressed block consisting of a mixture of a metal fuel material (such as aluminum powder) and a metal oxide (such as iron oxide powder) is lowered to a position in the well with some form of rigid platform 2 (such as a bridge plug) supporting it. The platform 2 can be either granular backfill material, cement, or a mechanical plug (bridge plug) protected by an insulating material (not shown) on its upper surface. The thermite charge 4 is then ignited by an electrical means, shown as igniter 3. The igniter can be located on the bottom, in the interior, or on the top of the thermite charge. As shown in FIG. 1B, after the igniter 3 is activated, the thermite charge 4 burns in place as a self-sustaining exothermic reaction. The burning occurs in a reaction zone 10 which propagates upward through the thermite charge 4. As shown in FIG. 1C, after the burning is complete the thermite charge forms a rigid hot plug 12 of a metal and oxide, ceramic-like material. The heat of the reaction may melt into the borehole wall material as shown at 14 in FIG. 1C, or melt through the steel well casing if one is present, to form a platform or sealing component.